Recently, an automatic document feeder is used for successively feeding many documents at a time. The common documents to be fed include slides, plain papers and photo papers, which are arranged in ascending order of thickness. As known, the feeding performance of the automatic document feeder is mainly dependent on document types. For recognizing and distinguishing different document types, an automatic document feeder having a mechanism for recognizing document types has been developed.
Referring to FIG. 1, a schematic cross-sectional view of a conventional automatic document feeder is illustrated. The automatic document feeder 4 principally includes an input tray 5, a pick-up roller 6 and a separation roller assembly 8. A document P is placed on the input tray 5. The input tray 5 has a perforation 5a. The pick-up roller 6 and the separation roller assembly 8 are disposed beside the input tray 5. In addition, a light emitter 13 and a light receiver 14 are disposed on opposite sides of the perforation 5a. The light receiver 14 is electrically connected to a control unit 50.
Generally, after the document P to be fed into the automatic document feeder 4 is placed on the input tray 5, the pick-up roller 6 transports the document P forwardly into a feeding path (not shown). The use of the separation roller assembly 8 assures that a single piece of document is picked to feed into the feeding path. In addition, the perforation 5a, the light emitter 13, the light receiver 14 and the control unit 50 cooperate to discriminate the type of the document P. The light emitter 13 may emit a light beam having several different intensities.
Before the document P is fed into the feeding path of the automatic document feeder 4, a single high-intensity light beam generated from the light emitter 13 successively penetrates through the document P and the perforation 5a, and is then received by the light receiver 14. During the high-intensity light beam penetrates through the document P, a small portion of energy contained in the high-intensity light beam is absorbed by the document P and another small portion of the high-intensity light beam is reflected by the document P. Consequently, the energy of the high-intensity light beam is diminished upon being received by the light receiver 14. Moreover, as the thickness of the document P is increased, the energy of the high-intensity light beam received by the light receiver 14 is reduced. Afterwards, the type of the document P is discriminated by the control unit 50 according to the energy of the high-intensity light beam received by the light receiver 14. Generally, the amount of the light beam energy is converted into a corresponding voltage value.
For discriminating the type of the document P, a series of experiments concerning the relationships of voltage values and document types are carried out to obtain a look-up table in advance. The look-up table is then built in the control unit 50. Take three document types (e.g. a slide, a plain paper and a photo paper) for example. In a case that the voltage value of the light beam received by the light receiver 14 is ranged from a first threshold value to a second threshold value, the document P is deemed as a slide. In another case that the voltage value of the light beam received by the light receiver 14 is ranged from the second threshold value to a third threshold value, the document P is deemed as a plain paper. In a further case that the voltage value of the light beam received by the light receiver 14 is ranged from the third threshold value to a fourth threshold value, the document P is deemed as a photo paper.
In some instances, the intensity of the light beam generated from light emitter 13 is adjustable. For example, if the high-intensity light beam penetrates through a very thin document (e.g. a slide), the energy of the high-intensity light beam which is absorbed and reflected by the thin document is very tiny. As a consequence, the energy of the high-intensity light beam received by the light receiver 14 is very close to that issued from the light emitter 13. Under this circumstance, the control unit 50 fails to accurately distinguish the document type. For solving this problem, the intensity of the light beam should be adjusted to a lower level. Similarly, after the low-intensity light beam penetrating through the thin document is received by the light receiver 14, the control unit 50 may analyze the energy of the low-intensity light beam received by the light receiver 14 so as to discriminate the document type.
Although the conventional automatic document feeder 4 has the function of discriminating document types, there are still some drawbacks. For example, since the light beam energy needs to be precisely analyzed, the circuitry of the control unit 50 becomes more complicated and thus the control unit 50 is not cost-effective.
Therefore, there is a need of providing an automatic document feeder having a mechanism of automatically discriminating the document type in a simplified and cost-effective manner.